Hierarchical establishment of information sources during foraging decision-making process involving Acromyrmex subterraneus (Forel, 1893) (Hymenoptera, Formicidae)

ABSTRACT During foraging, worker ants are known for making use of many information sources to guide themselves in external environments, especially individual (memory) and social (trail pheromone) information. Both kinds of information act in a synergic way, keeping the foraging process efficient and organized. However, when social and individual information is conflicting face a trail bifurcation, it is necessary to establish a hierarchical order so prioritizing one of them. This study aims to verify which information (social or individual) is prioritized by Acromyrmex subterraneus workers when facing a bifurcation in a Y-trail system. Only one branch of the Y-trail leads to food resource and it had a section covered by filter paper where trail pheromone was deposited by workers. Pheromone deposition was here estimated by worker flow. After an individually marked forager (target-worker) made 1, 3 or 5 trips to the food resource, the filter paper was transferred to the branch which did not lead to the food. The time spent by target workers on branch selection and their right choice (branch with food) frequency were registered. Regardless of the target worker's previous trips to the resource, right choice frequency stood over 70%. In addition, the number of previous trips did not influence the time spent on decision making. However, the higher the flow of workers, the longer the time spent on decision making. By simulating a situation with conflicting information, it was possible to verify that a hierarchical order is established by A. subterraneus, which prioritized individual information (memory).

Effect of the presence of brood and fungus on the nest architecture and digging activity of Acromyrmex subterraneus Forel (Hymenoptera, Formicidae)

ABSTRACT This study investigated the stimuli that trigger digging behavior in Acromyrmex subterraneus during nest building. The hypothesis was that the presence of the fungus garden and/or brood triggers the excavation of tunnels and chambers. For the experiment, the excavation rate of individually marked workers kept in plastic cylinders filled with soil was recorded. Four treatments were applied: (1) 30 medium-sized workers, 5 g fungus garden and 30 brood items (larvae and pupae); (2) 30 medium-sized workers and 5 g fungus garden; (3) 30 medium-sized workers and 30 brood items; (4) 30 medium-sized workers without fungus and brood. After 24 h, morphological parameters of nest structure (length and width of the chambers and tunnels in cm) and the volume of excavated soil were recorded. In contrast to the expected findings, no change in morphological structure, rate of excavation by workers, or volume of excavated soil was observed between treatments, except for tunnel width, which was greater, when no brood or fungus garden was present. Thus, the results do not support the hypothesis that the fungus garden and/or brood are local stimuli for nest excavation or that they mold the internal architecture of the nest. Although this hypothesis was confirmed for Acromyrmex lundii and Atta sexdens rubropilosa, the same does not apply to A. subterraneus. The digging behavior of workers is probably the result of adaptation during nest building in different habitats.

Dynamics of the restoration of physical trails in the grass-cutting ant Atta capiguara (Hymenoptera, Formicidae)

ABSTRACT Dynamics of the restoration of physical trails in the grass-cutting ant Atta capiguara. Leaf-cutting ants of the genus Atta build long physical trails by cutting the vegetation growing on the soil surface and removing the small objects they find across their path. Little is known on the dynamics of trail construction in these ants. How much time do they need to build a trail? To answer this question we selected six trails belonging to two different nests of A. capiguara and removed on each trail a block of soil of 20 cm × 15 cm that included a portion of the physical trail. This block was then replaced by a new block of the same size that was removed in the pasture near the trail and that was uniformly covered by the same type of vegetation as that found on the block of soil that was removed. The time required to restore the trail was then evaluated by the length of the grass blades found along the former location of the trail. The results show that ants rapidly restore the portion of the physical trail that was interrupted, which suggests that they could also do the same after their trails have been recolonized by the vegetation.

Conteúdo energético das operárias da formiga cortadeira, Atta sexdens / Workers energy content in leaf-cutting ants, Atta sexdens

O conhecimento do conteúdo energético das operárias é essencial para mensurar a disponibilidade energética, responsável pela manutenção e execução de atividades essenciais para o crescimento da colônia de formigas. Mas pouco se conhece sobre o conteúdo energético das operárias e quanto estará disponível para realizar atividades de, por exemplo, escavar do ninho. O presente estudo determinou o teor de lipídeo e o conteúdo energético das operárias de formigas cortadeiras inativas, antes e pós atividade de escavação. Por meio da determinação do teor de lipídeos, pode-se calcular o conteúdo energético das operárias em repouso (paradas) e compará-las com as que escavaram. O teor de lipídeo e conteúdo energético das operárias foram em média 9,1±0,8% e 111,31±54,71J, respectivamente, entretanto, as séries experimentais não diferiram significativamente. Adicionalmente, a taxa do fluxo catabólico, embasado na massa corporal das operárias foi de 14,76±10,11µW. Conclui-se que o recurso energético para a escavação do ninho não é proveniente de reserva de lipídeos corporal e, dessa forma, o conteúdo energético das operárias não alterou antes e pós-atividade de escavação.

The knowledge of the workers energy content is essential to measure the energy availability for maintenance and performance of activities essences for the colony growth. But little is known about the workers energy content, and how much will be available to carry out activities, for example, the excavation of the nest. The present study determined the lipid content and energy content of the worker ants inactive before and after excavation activity. Through the lipids determination, it can be calculated the energy content of workers resting (standing) with those which excavated. The lipid content and energy content of the workers were on average 9.1±0.8% and energy content of the workers was on average 111.31±54.71J, respectively, however, the experimental series did not differ significantly. Additionally, catabolic flux rate based in workers body mass was 14.76±10.11µW. It was concluded that the energy resource for the excavation of the nest is not coming from reserve lipid body, and thus the energy content of the workers did not change before and after excavation activity.

Influência do ambiente no desenvolvimento de colônias iniciais de formigas cortadeiras (Atta sexdens rubropilosa) / Influence of environment on the development of initial colonies of leaf cutting ants (Atta sexdens rubropilosa)

Anualmente, as colônias maduras de Atta spp. produzem formigas aladas, machos e fêmeas, que abandonam a colônia na qual foram produzidas para formarem novas colônias e, dessa forma, continuarem a perpetuação da espécie. A fundação de colônias de Atta spp. sempre envolve grande esforço da rainha, única responsável pelo cultivo do fungo e pelos cuidados consigo mesma e com sua prole. Nesse período de fundação ocorre o consumo de grande parte de suas reservas. O presente estudo investigou como o ambiente influencia o desenvolvimento das colônias recém-fundadas de Atta sexdens rubropilosa. Para tanto, foram comparadas colônias de campo coletadas cinco meses após a revoada e colônias mantidas em laboratório desde o voo nupcial. Foram analisados o peso e o tamanho da população (número de operárias, de larvas, de pupas e de ovos), o tamanho das operárias, o volume e peso do jardim de fungo e o esforço para a fundação dos ninhos (teor de lipídeos e peso das rainhas). Comparadas às colônias de campo, as colônias de laboratório apresentaram maiores valores no tamanho e no peso da população, no volume e no peso do jardim de fungo. As medidas das operárias, por classe de tamanho, não variaram entre as colônias de campo e de laboratório. O esforço da fundação dos ninhos foi maior nas colônias de campo, cujas rainhas apresentaram menor porcentagem de lipídeos e menor peso em relação às colônias de laboratório. Esses resultados demonstram que o ambiente exerce forte influência no desenvolvimento das colônias durante a fase de fundação, bem como sobre o status fisiológico das rainhas, sendo, portanto, a sobrevivência, o desenvolvimento e o sucesso da colônia reflexos das condições do ambiente e do esforço da rainha.

Annually, mature colonies of Atta ants produce winged males and females, where leave the colony in which they were produced to form new colonies and thus continue the species perpetuation. The foundation of Atta spp. colonies always involves high queen effort, solely responsible for growing the fungus and the care with itself and its offspring. During the foundation there is consumption of body reserves. This study investigated how the environment influences the development of newly founded colonies of leaf cutting ant, Atta sexdens rubropilosa. Therefore, it was compared field colonies collected five months after the nuptial flight and colonies maintained in the laboratory since the nuptial flight. It was analyzed the weight and size of the population (number of workers, larvae, pupae and eggs), the workers size, the volume and weight of the fungus garden and queen effort for the nest foundation (lipid content and weight of queens). In comparison to the field colonies, the laboratory colonies showed higher values in the weight and size of population, the volume and weight of the fungus garden. Measures of workers by class size did not vary between colonies from field and laboratory. The queen effort for nest foundation was higher in field colonies, which showed a lower percentage of fat and lower weight compared to laboratory colonies. These results show that the environment has a strong influence on the colonies development during the foundation, as well as on the physiological status of the queens, and therefore the survival, development and success of the colony, it is a reflection of the queen efforts and of the environment conditions.

Attractiveness of copperleaf-based bait to leaf-cutting ants / Atratividade de iscas à base de acalifa para formigas cortadeiras

Leaves of copperleaf (Acalypha spp.) are greatly accepted by leaf-cutting ants and therefore used to maintain colonies in the laboratory. In order to determine the attractive potential of granulated bait of Acalypha wilkesiana leaves versus citrus pulp, currently used in commercial baits, an evaluation was performed, by using the leaf-cutting ant Atta sexdens rubropilosa as model. The handmade non-toxic baits contained powder (90 percent w/w) of either citrus pulp or A. wilkesiana were offered simultaneously in the foraging arena until the total loading of one of them. Results showed that copperleaf baits are less attractive than the citrus ones, which invalidates the viability of the substitution.

Folhas de acalifa (Acalypha spp.) são de grande aceitação pelas formigas cortadeiras e, por isso, utilizadas na manutenção de colônias em laboratório. Baseado nesses fatos, procedeu-se a uma avaliação da atratividade de isca granulada feita com folhas de Acalypha wilkesiana, usando a formiga cortadeira Atta sexdens rubropilosa como modelo, com o objetivo de determinar o seu potencial atrativo frente ao citros, atualmente utilizado na formulação comercial de iscas formicidas. Foram formuladas artesanalmente iscas atóxicas, contendo pó (90 por cento p/p) de polpa cítrica ou de A. wilkesiana. As iscas foram colocadas simultaneamente em uma arena de forrageamento até o carregamento total de uma delas. Os resultados mostram que a isca de acalifa é menos atrativa que a de citros, o que inviabiliza a substituição.